Final Report: Characterizing the Habitat of Slender-Horned Spineflower (Dodecahema leptoceras): Geomorphic Analysis by Ms. Yvonne Wood, Ph.D. Candidate ." .. • Department of Soils and Environmental Sciences University of California-Riverside Riverside, CA 92521 and Dr. Stephen G. Wells, Adjunct Professor, UC-Riverside and Executive Director and Research Professor Quaternary Sciences Center Desert Research Institute P.O. Box 60220 Reno, NV 89506 Prepared for" Mary Meyer, Plant Ecologist, Region 5 California Department of Fish and Game 330 Golden Shore Suite 50 Long Beach, CA 90802 Funded by the U.S. Fish and Wildlife Service Section 6 Funds Contract Number FG4632-R5 1.0 INTRODUCTION 1.1 Statement of Purpose The slender-horned spineflower (Dodecahema leptoceras) is a state and federally-listed endangered species found solely in Southern Califomia. It is threatened by extinction due to the rapid pace of development in this region. This plant is found associated with fluvial and alluvial fan sediments related to streams draining the Transverse (San Gabriel) and Peninsular (San Bernardino and San Jacinto) Mountain Ranges. Known occurrences of Dodecahema are not in active channels but are typically found on nearby stream terraces and alluvial fan surfaces which have been estimated to be the result of 100 year flooding events (M. Meyer, 1994, pers. comm.). Recorded populations are spatially distinct from one another and have shown no areal increase in historic times (M. Meyer, 1994, pers. comm.). Little is understood about this species' method of dispersal; and while seeds can be propagated under greenhouse conditions, the resulting plants produce few seeds. Thus, there is great concern about the future viability of Dodecahema leptoceras (DOLE) as increased alteration of its habitat occurs. The goal of this research project component is to provide an understanding of the geomorphic setting of the DOLE habitat sites, including the ages of the alluvial sediments which support populations of the slender-horned spineflower. This research project component primarily included reconnaissance of sites on selected fluvial drainages supporting these populations: the Santa Ana, Bautista Creek, San Jacinto Wash, Big Tujunga Wash, Bee Canyon, and Arroyo Seco fluvial systems. Detailed field studies were undertaken on four of these fluvial systems: Santa Ana, San Jacinto, Bautista, and Arroyo Seco. The results of this study are submitted separately, but they compliment the ecological research project component supervised by Dr. Edith B. Allen. I 1.2 Regional Overview of Study Sites The stream systems whose sediments support DOLE are similar in their source lithology and their fluvial geomorphology, as affected by a Mediterranean climate and an active tectonic regime. Both of these latter two characteristics promote increased sediment production and storage in mountain valleys. This sediment becomes available for deposition during flooding events. The observed stream deposits in the study channels most likely resulted from (a) climatic changes or severe weather events (i.e., increased sediment yield related to more moist climatic regimes or 100 year flood events); (b) tectonic processes (i.e., increased sediment yield associated with tectonic uplift, oversteepend terrain, and deeply fractured bedrock); or (c) combinations of these two forcing factors. 1.2.1 Climate A region's prevailing climate has a major effect on the natural vegetation and also strongly affects fluvial geomorphic processes. In Southern California's south coast basin, a semi- add Mediterranean climate periodically produces intense winter rain events which orographically impact steeply inclined mountain ranges resulting from active tectonics. These conditions, in turn, promote the accumulation of sediment on hillslopes and valleys during dry spells. This sediment becomes available for transport during intense rain. Primary precipitation occurring during the winter months and long, dry summers generally inhibits forest cover lower than 1500 meters except on some north facing slopes. Below this level, chaparral and coastal sage scrub communities predominate. 1.2.2 Tectonics The lowlands of Southern California's coastal region are rimmed by steeply-sloping mountain ranges with peaks of 3000 to 3500 m. Much of the area of these ranges is above 2000 m in elevation. An active, complex tectonic system composed of the San Andreas fault and associated faults controls most of the region's topography (Muhs, 1987) and impacts its fluvial geomorphology. Regions in faulted shear zones also provide increased amounts of sediment to stream systems and basin catchments. 1.2.3 Lithology The lithology (bedrock) of the region is varied. However, the four drainages studied are all predominately composed of coarse to fine grained plutonic and metamorphic clasts. Different lithologies have differing resistance to weathering, susceptibility to slope failure, and contribute differing particle sizes to fluvial systems. Additionally, the following three drainages all have Pleistocene sediments as source material in close proximity to the spineflower populations -- the Bautista and Soboba Beds near Bautista Creek and the San Jacinto River, and the Potato Sandstone near the Santa Ana River. 1.2.4 Fluvial geomorphology One major effect of these components -- active tectonics, a Mediterranean climate, and granitic and metamorphic lithologies -- is the production of large quantities of sediment for transfer to the valley bottoms. Mass movements -- such as soil slips, debris flows, mudflows, and earthflows -- may be of greater importance to stream systems in this region than most others because of the combined effects of climate and lithology (Muhs, 1987). Most of these mass movements occur on slopes holding sediment accumulated during dry spells after heavy rainfall events. They differ in particle size distribution and other features, but they have in common the deposition of sediment upon low gradients. Debris flow deposits form alluvial fans at mouths of tributaries and debris trains in and along trunk streams (Campbell, 1974). Geomorphic features of semi-arid and arid fluvial systems can differ significantly from the 'classic' floodplain of the eastern United States. Semi-add and add stream systems and their bedforms reflect flashy conditions where debris flows or highly concentrated flows impinge on channels. These stream systems produce alluvial fans, with sediment-rich flows leaving patchy deposits as gradients decrease at the front of mountain ranges. Channels in alluvial fans often avulse -- that is 'jump' to a new location -- over short periods of geologic time. Thus, significantly older sediments can be found abutting an active channel instead of the surface adjacent to an active channel being a recent floodplain. "The presence of compound channels and the absence of geomorphologic flood plains are significant differences that result from the operations of the add-regime hydroclimatic systems." (Graf, 1988). 4 2.0 METHODOLOGY 2.1 Introduction The primary goal of this project is to establish age control, where possible, for surfaces supporting DOLE. Methods of dating Holocene surfaces include: 1) numerical age dating including radiogenic methods such as _4C;2) relative age dating based on establishing spatial relationships between surfaces and then assigning positional ages or by comparing the physical effects of time on soil development between surfaces; and 3) correlative age dating in which a series of relatively established age sequences are tied to numerical time by correlation with another regional surface of known age. Material for absolute age dating is relatively rare on quaternary surfaces and can have constraints on its accuracy when found. As a result, a strong reliance is placed on the use of relative and correlative dating techniques -- such as soils, landscape position, degree of lichen growth, degree of surface smoothing, and relative clast integrity. The fluvial systems included in the scope of this study were investigated through: (1) a review of written materials containing geological and soils data; (2) interpretation and mapping of aerial photographs; (3) field observation of the surfaces; (4) radiocarbon dating; and (5) soil descriptions that allowed comparisons to regional dated surfaces. 2.2 Brief Descriptions of General Methods 2.2.1 Aerial Photographic Interpretation and Mapping Aerial photographs obtained from local flood control agencies and the UC Riverside aerial photography collection were analyzed through a stereoscope. From this analysis, maps were drawn of the relative topographic position of surfaces. These maps were then 'field-truthed' through on-site examination of surfaces. 5 2.2.2 Field Observations General field observations were made on trips to the sites including the morphologic character of the stream channel reaches, relations with terraces and surrounding hillslope colluvial wedges and alluvial fans. Detailed observations of surface microtopography, slope, clast weathering, and lichen cover were made for sites supporting DOLE. 2.2.3 Radiocarbon Dating Geomorphic surfaces which could be related stratigraphically to the spineflower surface in each drainage were searched for the presence of material which could be radiocarbon dated. Such material could include charcoal, charred wood, peat, or freshwater snail shells. The occurrence of such material is relatively rare. When found, this material must be examined for evidence of possible fluvial reworking